The guys over at the German heli shop Freakware.de did some great research on the minor tail oscillations that some reported on the new T-Rex 250.
These oscillations (which can also be observed on some of the online videos) can not be resolved by:
- a different gyro
- shortening the blades
- different mounting points of the gyro
- adding weights to the tail rotor blades
These things can help, however:
- fix the battery tightly
- check chassis screws
- increase the delay for the GP 750 (which basically works like Expo, but does not reduce servo speed)
- use a slower tail servo (not the best of all solutions, of course)
- the pointed blades are much better than the curved ones
- use harder adhesive pads to mount the gyro
The guys from freakware.de put great effort in researching the real cause for the oscillations. They finally figured out the mechanical cause using a high speed video camera with 600 frames/sec: The tail pivot arm twists under load. This absorbs some of the servos motion and stores its energy. When the gyro stops the servo motion to compensate the current oscillation, the pivot arm releases the stored energy by un-twisting, and the tail over-compensates. The gyro detecs this and the whole process starts over.
The good news about this is that you can also fix this by stiffening the tail pivot arm with carbon fiber roving (at a 45° angle would work best) and fix it with CA. Another thing that wouldn’t be a bad idea would be to add another tail rod guide (given that you can get your hands on some T-Rex 250 spares already).
Here’s the high-speed video they taped:
UPDATE Dec-21
Well, it looks like the bending of the tail pitch lever was just a symptom, not the cause. Here’s the original tail rotor hub set (Align part no. H25074):
…and now take a look at the NEW one (part H25074-1):
Notice the new set of washers that have been added. There have been other solutions reported, involving grinding of the tail rotor central piece, but most users report the new tail rotor hub set fixes the problem!
Good grief guys - Has anyone bothered to manually operate the tail pitch control rod whilst the heli is running (sans main blades of course) to see how much force is required to vary the pitch of the tail blades? I don’t mean the force needed to push the blades away from zero pitch but instead to move the pitch from one thrust setting to another. I just ordered a 250 T-Rex and will be able to investigate this issue on my own and if I see this same tail hunting problem, you can bet I’ll be checking to see if the tail can change pitch smoothly and reliably when the tail rotor is up to operating speed. I completely expect to see some binding in the tail grip assemblies as the root cause for this problem. If the grips bind, the force needed to make a pitch change will appear to be a flexing in the bellcrank when it’s really excessive drag in the grips. I’ve seen this very same thing happen when a tail isn’t assembled correctly and when the pushrod is worked by hand, the force needed is detected as clearly too high. I BET the T-Rex is suffering from similar symptoms - figure out why.
Steve